I prefer one basil plant by itself. I plant several in my garden and they get huge.

Basil seedlings are not people, puppies, or kitties. This is a totally renewable resource. . If you try to separate them and their roots are entangled....you are likely to damage them all. If they can be separated easily, you simply make a decision about how many to keep and how many to dispose of or give away. Composting is not a waste, it's a contribution.

Next time, you'll plan better and won't sow so many. I would only use one sturdy seedling per cell.

also, if you pick the biggest and best
when you save seed, you will be getting the cream of the crop
as far as the DNA goes
and have better plants next year.

When i have lots extra of seed and space, i plant a LOT of seed
like papaya (500 in a fruit)
and i keep just the best seedling for trees
odds are they will be healthier and stronger.